There are many applications which require the generation of a signal when a workpiece arrives at a predetermined location. Sensor arrangements for scanning flat, sheet like material workpieces as they progress through a sewing machine are known in many variations.
Some machines employ mechanical means for detecting the workpiece edge. Mechanical sensor arrangements, however, have been known to be particularly sensitive and/or delicate and are susceptible to malfunction. Also, mechanical sensors are subject to wear as well as vibration. Some mechanical sensors are too insensitive to detect the movement of a single ply workpiece and, thus, do not provide the reliability required in some operations.
In contrast to mechanical sensors, it has been known to use optical scanners or sensors for detecting the workpiece. Such optical sensors, however, also have drawbacks associated therewith. Optical sensors are usually sensitive to light dispersion and differing material density. Further, many optical sensors are sensitive to ambient light. Moreover, the environment in which this type of apparatus finds utility is usually laden with dust and lint, both of which may effect the efficiency of such sensors. Photocells are usually not employed in the immediate area of sewing because of the vibratory surroundings which prohibit their use. Furthermore, disturbances with light sensors may arise if the operator inadvertently interrupts the light beam, thus producing incorrect detection signals.
Some devices employ air sensors for detecting the workpiece. These also have drawbacks. As mentioned above, the environment in which these sensors find utility is laden with dust and lint. Many of the machines are also exposed to some lubricant, which, when combined with the dust and lint in the area, easily clog the air sensor arrangements, and thus effect the reliability of same.